1. Field of the Invention
The present invention relates to an apparatus for and a method of measuring capacitance with high accuracy.
2. Description of the Related Art
In the development of semiconductor devices and semiconductor fabrication processes, it has heretofore been customary to measure capacitance for the evaluation of the devices or processes. Capacitance is measured basically according to two principles: high frequencies measurement and quasi-static measurements. For quasi-static capacitance measurement, there has been a demand for a technique to measure capacitance with high accuracy because values of capacitance to be measured are small because semiconductor fabrication processes have become finer in recent years. Owing to the use of finer semiconductor fabrication processes, a large leakage current that causes errors in the measurement of quasi-static capacitance, and such a large leakage current needs to be compensated for.
Capacitance measurement processes that are primarily used in the art at present include a ramp voltage process and a step voltage process.
According to the ramp voltage process, a ramp voltage having a constant slew rate, which is a value produced by differentiating the voltage with respect to time, is applied to a capacitance to be measured, and the value of the capacitance is determined from a current I that is measured and the slew rate (dV/dt) according to the equation C=Q/V=Ixc2x7dV/dt. One significant shortcoming of the ramp voltage process is that it cannot compensate for any leakage current.
According to the step voltage process, a step voltage is applied to a capacitance to be measured. Specifically, a voltage applied to a capacitance to be measured is changed from a certain value V1 to another value V2 (xcex94V=V2xe2x88x92V1), and a current flowing through the capacitance over a certain period of time in which the voltage varies is integrated to determine a quantity AQ of electric charge that is stored in the capacitance depending on the voltage change. For example, leakage current L1 at voltage V1 and leakage current L2 at voltage V2 are measured. If it can be assumed that a leakage current in the integrating period is the average of leakage currents L1 and L2, then the capacitance Cx to be measured can be calculated according to the following equation:   Cx  =            {                        ∫                      i            ⁢                          ⅆ              t                                      -                              (                                                            L                  2                                +                                  L                  1                                            2                        )                    ⁢                      t            i                              }        /          (                        V          2                -                  V          1                    )      
where i represents the current value and ti represents the integrating time. The step voltage process is problematic, however, in that a transient current flowing when the voltage is changed may possibly exceed the dynamic range of the measuring system. The portion of the current that exceeds the dynamic range is responsible for a measurement error as it cannot properly be measured. Stated otherwise, different pieces of hardware need to be made available to handle various values of capacitance to be measured.
It is therefore an object of the present invention to provide an apparatus for and a method of measuring a capacitance with high accuracy by controlling a current flowing through the capacitance in view of the dynamic range of a measuring system in a step voltage process.
According to the present invention, there is provided an apparatus for measuring a capacitance, comprising a voltage source with a current limiting function for applying different voltage values to the capacitance, and an integrator capable of continuous integrating operation for repeatedly integrating a current flowing through the capacitance at constant periodic intervals. The integrator capable of continuous integration operation can output an integrated value for a certain period, resets the integration operation after having outputted the integrated value, and output an integrated value again after a next certain period. The integrator is capable of repeating the above operation. The voltage source with a current limiting function can limit the output voltage depending on a given current value.
The voltage source may comprise a variable-output voltage source, a variable-output current source, and a current detector which is selectively connectable to the variable-output voltage source and the variable-output current source depending on the value of a current flowing therethrough.
According to the present invention, there is also provided a method of measuring a capacitance, comprising the steps of imparting a voltage change from a voltage value to another voltage value to the capacitance while limiting a current flowing through the capacitance so that the current does not exceed a predetermined value, repeatedly integrating a current flowing through the capacitance at given periodic intervals during the voltage change and outputting integrated values, summing the outputted integrated values to determine a current value flowing through the capacitance in relation to the voltage change, and calculating a value of the capacitance based on the calculated current value.